-LRB- CNN -RRB- -- A 15-year-old student shot and killed a teacher inside a classroom in southern Estonia on Monday , police said .

There were four other students inside the classroom , but no one was injured , police spokeswoman Tuuli Harson said .

Investigators are working to determine how the teen got the revolver he used , and why he opened fire , she said .

`` School personnel took away the gun and the boy was handed over to the police . All the students are safe . All the children have left school and are with their families , '' she said . `` The police have checked all the floors in the building and nothing suspicious was found . ''

Estonia Prime Minister Taavi RÃµivas described the shooting , which occurred in the town of Viljandi , as a shock for the nation .

`` We do not yet know the causes of this incident , but violence is never an acceptable solution . Such things should not happen , '' RÃµivas said in a statement . `` We all have a duty to notice a person who is in need of support in struggling with his or her problems . Today 's incident is a painful reminder of that . ''

CNN 's Catherine E. Shoichet and Emil Hellerud contributed to this report .

@highlight

A school shooting happens in Viljandi , Estonia

@highlight

Police say a 15-year-old student shot and killed a teacher

@highlight

Estonia 's Prime Minister describes the shooting as a shock